1. Field of the Invention
The present invention relates to the structure of a combustion chamber for an in-cylinder direct fuel injection type spark ignition engine and particularly to the structure of a combustion chamber suitable for stratified charge combustion.
2. Prior Arts
Many approaches have been proposed to improve fuel economy of vehicles and in particular an improvement of theoretical thermal efficiency, a reduction of pumping loss, a reduction of friction and the like have been proposed to raise fuel economy of an engine. To improve the theoretical thermal efficiency or to reduce the pumping loss, there are various approaches for accomplishing a lean burn control, a high EGR rate combustion and the like, besides conventional approaches such as raising a compression ratio or an expansion ratio. In making these approaches, miscellaneous gas flow control techniques such as so-called tumble, swirl or squish are essential to improve the combustion of mixture gas in cylinders.
These gas flow controls have been used originally for raising the combustion velocity and forming a homogeneous mixture gas in cylinders but on the other hand the flow control is purposely used for forming a locally rich mixture zone or forming a mixture gas in a particular area while the mixture gas remains in a lean condition as a whole. In this case, the combustion strategy is categorized in so-called stratified charge combustion but not in homogeneous charge combustion.
In order to realize stratified charge combustion more efficiently by making use of the flow control method, an in-cylinder fuel injection engine in which fuel is injected directly into cylinders is very effective.
For example, Japanese Patent Application Laid-open Toku-Kai-Hei No. 5-1544 discloses a technique in which a masking wall provided around the intake valve generates an inverse tumble flow forcedly so as to carry the fuel injected obliquely from the fuel injection means provided beneath the intake port on the tumble flow to a spark plug disposed in the center of the cylinder head.
Further, Japanese Patent Application Laid-open Toku-Kai-Hei No. 6-146886 discloses a combustion chamber in which the fuel injection means are disposed beneath the intake port in the same way as in Toku-Kai-Hei No. 5-1544 and the intake port has a sectional configuration with one half side thereof expanded. Due to this configuration with one half side expanded, the main stream of intake air is biased toward the cylinder wall, thereby the generation of the inverse tumble flow is fostered. Fuel injected obliquely with respect to the cylinder axis is carried on the inverse tumble flow.
Furthermore, Japanese Patent Application Laid-open Toku-Kai-Hei No. 6-42352 proposes a combustion chamber technique wherein the fuel injection means is disposed in the center of the cylinder head with the injection nozzle directed downward and the spark plug is projected from between two intake ports.
However, the technique disclosed in Toku-Kai-Hei No. 5-1544 has a problem of power shortage at high speeds and high load areas because of the masking wall provided around the intake valve.
Further, the combustion chamber disclosed in Toku-Kai-Hei No. 6-146886 has a defect that since the inverse tumble flow is formed only in one half portion of the combustion chamber and only the concave portion provided on the top surface of the piston forms a combustion space, the compression ratio becomes very high in a limited area of the combustion chamber. Hence, the engine must be designed so as to adjust to running on gasoline with high octane rating.
Further, according to this combustion chamber, since the distance between the injector nozzle and the electrode of the spark plug is so large that it is difficult to control the local air-fuel ratio in the vicinity of the electrode and this causes an unstable combustion and a low ignitability.
Further, according to above two disclosures, because fuel is injected obliquely with respect to the cylinder axis, there is anther problem that fuel sticks to the inner wall of the cylinder. Fuel sticking may exacerbate fuel economy due to excess cooling of the cylinder wall.
Further, the stuck fuel strips lubrication oil from the inner wall of the cylinder and this results in a poor lubrication between the piston and the cylinder.
Further, in the combustion chamber according to Toku-Kai-Hei No. 6-42352, the spark plug is disposed in the center of the cylinder head with the electrode oriented directly downward. Further, the electrode of the spark plug is arranged so as to expose to the fuel sprayed from the fuel injection means. Therefore, this arrangement is aimed at igniting the end portion of injected fuel, then gathering the rich mixture to the center of the combustion chamber and completing combustion therein, rather than igniting fuel reflected on the top surface of the piston.
However, this combustion chamber has a problem that a good stratified charge combustion can be obtained in the condition where the amount of fuel injection is small but it is difficult to suppress the generation of smoke in the condition where the amount of fuel injection is increasing.